AbstractTo characterize clinical and laboratory signs of patients with Still’s disease experiencing macrophage activation syndrome (MAS) and identify factors associated with MAS development. Patients with Still’s disease classified according to internationally accepted criteria were enrolled in the AutoInflammatory Disease Alliance (AIDA) Still’s Disease Registry. Clinical and laboratory features observed during the inflammatory attack complicated by MAS were included in univariate and multivariate logistic regression analysis to identify factors associated to MAS development. A total of 414 patients with Still’s disease were included; 39 (9.4%) of them developed MAS during clinical history. At univariate analyses, the following variables were significantly associated with MAS: classification of arthritis based on the number of joints involved (p = 0.003), liver involvement (p = 0.04), hepatomegaly (p = 0.02), hepatic failure (p = 0.01), axillary lymphadenopathy (p = 0.04), pneumonia (p = 0.03), acute respiratory distress syndrome (p < 0.001), platelet abnormalities (p < 0.001), high serum ferritin levels (p = 0.009), abnormal liver function tests (p = 0.009), hypoalbuminemia (p = 0.002), increased LDH (p = 0.001), and LDH serum levels (p < 0.001). At multivariate analysis, hepatomegaly (OR 8.7, 95% CI 1.9–52.6, p = 0.007) and monoarthritis (OR 15.8, 95% CI 2.9–97.1, p = 0.001), were directly associated with MAS, while the decade of life at Still’s disease onset (OR 0.6, 95% CI 0.4–0.9, p = 0.045), a normal platelet count (OR 0.1, 95% CI 0.01–0.8, p = 0.034) or thrombocytosis (OR 0.01, 95% CI 0.0–0.2, p = 0.008) resulted to be protective. Clinical and laboratory factors associated with MAS development have been identified in a large cohort of patients based on real-life data.